The 41st annual meeting of the Division of Planetary Sciences takes place October 4-9 in Fajardo Puerto Rico. The DPS is a division of the American Astronomical Society. About 700 scientists and students have congregated here in Fajardo to present the latest results from their research on the solar system, ranging from Mercury to the outer solar system and extrasolar planets. Tomorrow we have sessions on the icy satellites of the giant planets, exoplanets, and first results from the Moon Mineralogy Mapper on the Indian Chandrayaan-1 spacecraft. Also on Monday we have an open forum meeting (town hall style, without the Hitler references, though) on the decadal surveys. These surveys, conducted by the National Research Council at the request of NASA, identify the leading scientific questions in the various fields of interest to NASA and become useful roadmaps in the following decade for prioritizing missions and exploration programs.

It looks like it will be a very full week: in addition to being an author on 5 papers here, I’ll be chairing a session, serving on two committees that are meeting here, including DPS’s governing committee, as well as meetings to discuss various scientific collaborations.

Another cool sequence of images of shadows as Saturn approaches its northern spring equinox in just a few months. This time, we see the shadow of the moon Epimetheus moving across the outer A ring. The wide gap is the Encke Gap, about 330 km across, while the narrow gap near the outer edge of the ring is the Keeler Gap (about 30 km across). Epimetheus shares an orbit with the moon Janus. They each orbit Saturn roughly 6 times for every 7 orbits of a particle at the outer edge of the A ring. This orbital resonance between the moons and the ring edge controls the structure and evolution of the edge.

As equinox approaches, the shadows will get longer, and we will start to see shadows cast by warps and distortions within the rings themselves. NASA’s full press release is here.

Just another stunning example of the beautiful compositions that arise from the combination of Saturn and its rings when you have a spacecraft that can view it from a variety of angles. With Saturn’s equinox approaching, the rings are getting dark, but it still makes a difference whether we are seeing the lit side (currently south) or the unlit side, as in the image below. The opaque B ring is completely dark, while the inner translucent C ring allows light through. Though we are seeing the night side of the planet (as can be seen from the shadow line of the planet on the rings, the Sun is to the left), the southern hemisphere is fairly well illuminated by light reflecting off the southern face of the rings. The northern hemisphere of the planet is much darker because it is only illuminated by light that makes it through the rings. The darkest line on the planet is the equator which gets virtually no light from the rings.

Cassini is now well into the “Cassini Equinox Mission”, a two-year mission following the completion of its successful four-year prime mission last summer. The scientific theme driving the equinox mission is to observe seasonal changes throughout the system as Saturn has its vernal equinox (the moment when the Sun crosses Saturn’s equator, moving from south to north). The equinox occurs shortly after midnight (Universal Time) on August 11, 2009 (the evening of August 10 in American time zones). The actual moment of equinox is not critically important because of the finite size of the disk of the Sun as seen from Saturn and the relatively slow pace of Saturn’s orbit around the Sun (29.66 years for a Saturnian year). This means there is an equinox season of weeks to months during which the rings are nearly edge-on to the Sun. One aspect of this of interest to ring scientists is that any deviations out of the ring plane will result in long shadows on the rings. By measuring the shadows we can get very accurate measurements of these small vertical perturbations which are otherwise hard to see. An early look at this sort of phenomenon is visible in the picture below in which the shadow of one of Saturn’s moons is clearly visible on the ring. The extent of the moon shadow, combined with our already precise knowledge of the relative positions of the Sun, Saturn, and the moon, will enable imaging team scientists to better constrain the inclination of the orbit of the moon. Soon, small ripples in the rings will cast shadows and, in a sense, become visible for the first time.

Saturn’s moon Epimetheus casts a shadow on the outer portion of the A ring as Saturn nears equinox. Image: NASA/JPL/SSI

I’ve had the mixed pleasure of spending a fair amount of time experiencing what is usually called “weightlessness”. I say it is a mixed pleasure because while the sensation of weightlessness is amazing and so different from our everyday experience of the world, I have experienced it on parabolic airplane flights which have the unhappy side effect in a segment of the population of inducing nausea and vomiting. I am in that unlucky segment. The body does adapt, and my last flight was puke-free. Other names used to describe the state of weightlessness are zero-g, no gravity, microgravity, and freefall. The latter is the only one that is truly accurate.

As an astronomer, gravity is the force that most concerns me professionally, and it is also the force that most of us have the most direct intuitive relationship with in our daily lives. And yet the relationship between gravity and freefall or “weightlessness” seems to be as elusive to most people as the sensation itself. Whether I am lecturing to a university astronomy class, speaking to a group of elementary school kids, or giving a public lecture to educated professionals, I always try to demonstrate the amazing insight of Isaac Newton about gravity: the same force that makes the Moon orbit the Earth is responsible for apples falling to the ground. While it is easy to understand those words, their implications for how the solar system works and for “weightlessness” usually remain abstract or obscure. Working against us is not just our daily experience (and, one could reasonably argue, millions of years of evolution), but also the language we use to describe gravity and its presumed absence.

Here is my standard gravity stump speech. For these purposes we do not need to stray into the exotic terrain of warped space-time and Einstein’s general relativity. Our sensation of gravity here on planet Earth comes not from the force of gravity exerted on us by the Earth, but by the competition between that force and all the stuff that gets in the way of it. If you are sitting now, you feel your weight because the chair is stopping you from falling to the floor. The actual sensation of weight I feel right now is due to pressure of a chair seat against the backs of my legs, the pressure of the floor against the bottom of my feet, and the pressure throughout my body produced by the weight of head on neck, torso on lower back, and so forth. So there are two ways to get rid of that pressure: get rid of the Earth, or get rid of the chair. If the chair beneath you were instantly snatched away, you would fall to the floor. And in that split second you would not feel the pressure of the chair on your backside. That sensation of weight would be gone, even though the Earth’s gravity is still very much present.

How about the weight of your head on your neck, etc? Galileo’s famous experiment at the tower of Pisa gives us the answer. Here again, though we may be familiar with the facts of the experiment, the implications are difficult to internalize: gravity makes everything fall at the same speed, whether it be a feather or a hammer, a head or a body. We (and centuries of thinkers between Aristotle and Galileo) have a hard time with this because air does a better job of slowing a feather than it does of slowing a hammer, so, in fact, the feather does fall slower. But if you get rid of the air (easy enough in a small lab experiment), they all fall at exactly the same rate. So when that chair is snatched away, all parts of your body will fall toward the floor at exactly the same rate. There will be no pressure of any part pushing up against any other part. And since that pressure is what we experience as weight, its absence gives us, in that brief period before slamming into the floor, “weightlessness.”

And yet we are still experiencing the Earth’s gravitational pull. In fact, in physics the term “weight” refers not to the pressure we feel from the chair, but simply the force of gravity acting on an object. Removing the chair does nothing to alter that force. It removes instead what is called the “normal force” of the chair that exactly cancels the force of gravity acting on our bodies. The rigid structure of the chair exerts an upward force on our bodies that keeps us from moving down due to the force of gravity. One might then consider the sensation we experience when the chair disappears not to be weightlessness, but normallessness.

I don’t think that will catch on.

We usually associate “weightlessness” with the image of astronauts “floating” inside a spaceship. This gives the impression of motionlessness (I’m going to see how many words I can add “lessness” to). However, it is the very large motion of these astronauts that makes them “weightless”. They are in a spaceship that is falling toward the Earth. There is no chair holding it up. And because the spaceship and the astronaut (like the hammer and the feather) fall toward the Earth at the same rate, the astronaut does not move relative to the spaceship. She appears to float inside it, yet there is nothing holding her up. Both she and the spaceship are falling freely toward the center of the Earth. Happily, they will not hit the Earth because previously, rockets accelerated the spaceship to such a high speed that by the time it has fallen the distance needed to hit the Earth, it has zipped over so much of the Earth that the curvature of the Earth has made the surface that much further away from the spaceship again. Here, then, is the similarity between the apple and the Moon that Newton recognized: the Moon is falling toward the Earth, but because of its great speed, it keeps missing the Earth.

An orbiting object such as the Moon or the space station is simply falling toward the Earth, but missing it.

So the only connection between space (as in “outer space”) and weightlessness is that getting above the atmosphere is the easiest way to fall for a very long time without running into something. But the exact same thing happens (for a very short time) when you snatch the chair out from under someone. So, “weightlessness” can be achieved by finding a way to fall for an extended period of time without any slowing due to air friction or, preferably, uncomfortably hard landings. Parabolic airplane flights accomplish this by flying the same path that an object falling toward the Earth would follow if there were no atmosphere. Because this is easily calculated, pilots can fly planes on such paths. While they do so, everything inside the plane follows the path than an object falling toward the Earth would follow if there were no atmosphere. So the airplane seat is falling as fast as you are, and it therefore doesn’t push up on you. Your arms are falling as fast as your shoulders, so they do not pull down on you either. You experience “weightlessness” because you are falling freely very quickly. The pilots make sure to achieve crashlessness (okay, that’s a stretch) on the flight by having the plane pull up before it heads toward the Earth too quickly. When it does, your body wants to head toward the Earth quickly, but the plane is rudely interrupting that fall and exerts a pressure against you that is much greater than normal. We thus feel heavy or excessive weight.

In fact, you are, when “weightless” accelerating at 1-g, where g is 9.8 meters per second per second. Right now, sitting on a chair in a normal terrestrial environment, your acceleration is zero-g. Weightlessness is really motion at 1-g, and not zero-g. The net force acting on us when we feel heavy is zero, while the net force acting on us when we feel weightless is equal to the local force due to gravity.

What is a planet? This seems to be an embarrassing question for a planetary scientist to be asking, but since the International Astronomical Union (IAU) passed a resolution in summer 2006 defining a planet, it has become a topic of increasing discussion and some controversy. At the 40th annual meeting of the Division for Planetary Sciences (DPS) of the American Astronomical Society being held this week at Cornell University in Ithaca, New York, there was even a special session devoted to the topic.

Earlier in the week I attended a talk by Dr. Alan Stern, Principal Investigator of the New Horizons mission to Pluto, at the University of Central Florida where he was visiting our research group. Stern made an eloquent case for ditching the IAU’s planet definition in favor of one that is more inclusive. Yesterday, at the DPS meeting here in Ithaca, I heard an equally eloquent argument by Stern’s colleague Dr. Hal Levision of the Southwest Research Institute defending the IAU definition.

The IAU definition says, in short, that a planet is an object that is large enough to be round and that orbits a star and which has “cleared” its orbit. Large enough to be round means, for all practical purposes, about 500 km in diameter. Stern would leave it at that. That is, he favors a definition of planet based on the intrinsic properties of the object independent of where it is in the universe and what effect it has had on other objects. With this definition, the geophysical planet definition, the Moon is a planet, as are Pluto, the large moons of Jupiter, many objects in the Kuiper Belt as well as Ceres, the largest asteroid. The geophysical planet definition makes no requirement on a planet orbiting a star instead of orbiting another planet or indeed drifting through interplanetary space. It simply must not be a star, which is defined as an object that now or in its past has undergone some form of nuclear fusion in its interior, and must be large enough to be in hydrostatic equilibrium (that is, roughly round due to the force of gravity overcoming its internal strength). With this definition, there are hundreds, perhaps thousands of planets in our solar system, depending on the unknown population of large objects in the Kuiper Belt.

Perhaps the main problem with the geophysical planet definition, as pointed out by Levison at DPS, is that it does not provide a clean break between planets and non-planets. In any planetary system, including our own, there will be objects just smaller than the minimum size for hydrostatic equilibrium that are neighbors of “planets”. So, there could be a spherical object – a planet – in the Kuiper Belt, and another object virtually identical in every way and in nearly the same orbit, that does not meet the “round” criterion and so would not be a planet. This seems arbitrary.

The dynamical definition, the one adopted by the IAU, requires that the object gravitationally dominate its particular region of the solar system. The awkwardness of this definition is that if one could move Earth sufficiently far from the Sun, it would no longer have the gravitational influence necessary to clear its new much larger orbital zone and would not be a planet. Conversely, Pluto would be a planet under this definition if it were much closer to the Sun. The advantage to this definition is that if one looks at the distribution of objects in the solar system, taking into account their sizes and their orbits, there is a very clear break between “planets” and small solar system bodies and “dwarf planets” (the latter being objects that meet the round and star-orbiting criteria of the IAU, but not the orbit-clearing one).

The sizes of the spots are proportional to the sizes of the planets. The vertical axis shows the orbital eccentricity of objects, while the horizontal axis shows their average orbital distance from the Sun. The four terrestrial planets (Mercury, Venus, Earth, and Mars) and four giant planets (Jupiter, Saturn, Uranus, and Neptune) stand out clearly from the other objects.

To avoid the problem posed by this dynamical evolution definition (that, for example, Earth would not be a planet if further from the Sun, or in fact, if it orbited Jupiter), Levison proposes modifying the definition so that instead of having one based on clearing an orbit, it is based on not being part of a continuous size distribution of objects in its region of the solar system. Thus, Jupiter in the plot above is not the only object in its orbit (so it has not technically “cleared” its orbit), but there is no object near its size. With this size distribution definition, “planet” refers to objects that are distinctly larger than any other objects in their orbital zone. It also leads to a distinct group of 8 planets in our solar system.

Over the next several years, this question will probably be resolved by the evolution of common public and professional usage rather than new IAU definitions. In the meantime, though, it has provided some interesting discussion on the apparently basic topic of what it means to be a planet.

On Monday Cassini had its closest encounter to date with the intriguing moon Enceladus, which spews water vapor from a number of points along the infamous “tiger stripe” fissures over its south pole. The closest approach to the Moon was up near the equator, and the trajectory of the spacecraft took skimming along the perimeter of the boundaries of Enceladus’s geysers. Turning back to look at the south pole as it flew away, Cassini had to maneuver quickly to capture clean pictures. The imaging team has now been able to piece together and “navigate” the images, meaning they have figured out where on Enceladus each image is. Tying this together with earlier work by Joe Spitale, Carolyn Porco, and others on the imaging team that identified various jets from the tiger stripes, they have now given us the first close-up views of two source regions.

The Damascus Sulcus region of Enceladus, with the location of two geysers indicated.